The present invention is a lifting or elevator device adapted to lift one end of a unit package so that a slip sheet can be placed thereunder. After such placement, the unit package is dropped and the elevator tilts out of the way so that the unit can be handled by a push/pull lift truck.
Push/pull lift trucks are widely used throughout industry since they do not require their loads to be on pallets, instead, the load is placed on a pull or slip sheet with a pull tab, typically 10-15 cm long, left protruding from one or more edges of the load. A push/pull lift truck is a modification of a conventional fork lift. The forks are much wider, and may be approximately equal to the full width of the loads being handled. They are also thinner at the tips to simplify insertion under the pull tab. The principal difference is the addition of a clamp unit for grasping the pull tab of the slip sheet and a ram for ejecting the load when it has reached its ultimate destination. The pull tab clamping mechanism is mounted at the bottom edge of the ram, immediately above the broad forks. In use, the truck operator will approach a load and slip the leading edge of the forks under the pull tab. The ram/clamping mechanism is moved to a forward position, where the clamp grasps the pull tab. The mechanism is then retracted, drawing the load onto the forks. At the point of delivery, the procedure is repeated in reverse order.
A frequent problem with the use of a push/pull lift truck is that of insertion of the slip sheet under the load being handled. The load, or at least one edge of it, must in some way be elevated above the holding surface so that the slip sheet can be inserted. The present invention is directed to this end. One method used in the past has been to drive the tips of the forks under one edge of the load and lift it sufficiently so that the slip sheet can be put in place. This is both awkward and hazardous. Injuries have occurred when a load slipped off the forks and struck the operator.
One industry in which push/pull lift trucks are widely used is the manufacture of corrugated shipping containers. Few plants are sufficiently conveyorized so that unit load handling is not required. The first place where such handling is needed is at the end of the corrugator. Shipping container blanks are assembled into large units and then delivered either to inventory or directly to printing and box-manufacturing machinery. The problem is particularly acute in plants where the unit packages of box blanks are assembled off the corrugators on down-stackers. This is an apparatus in which the blanks shoot off the end of the corrugating machine onto the top of a pile which is being continually lowered as the unit package is assembled. In most plants having down-stackers, it is difficult, if not impossible, to insert the slip sheet under the bottom of the unit package before it is assembled. After the unit is assembled, it normally moves down a roll case type conveyor from which it is then moved to inventory by a push/pull lift truck.
The prior art does not appear to have addressed the problem of safe and convenient placement of slip sheets under loads to be handled by push/pull lift trucks. While a large number of elevator-type devices are available on the market, none of them appear satisfactory to handle the immediate problem. As an example, the patent to Cestone et al., U.S. Pat. No. 3,680,837, shows a beam supported by a jack at each end with the load suspended beneath the beam. Conceivably, a device of this type could be modified to lift one edge of a package for insertion of a slip sheet. This would have to be placed beneath the beam when the package was elevated. Unfortunately, when the jacks and beam are lowered, the beam still remains located over the slip sheet, unless the jacks have a very long throw so that the pull tab can be freed by bending it down as the jacks descend. This is not a very satisfactory alternative, since very often space is quite limited, so that long travel of the jacks is not possible. Additionally, the tab is often permanently deformed so that it is difficult to grasp by the clamping mechanism on the push/pull truck.
Another elevator mechanism is shown in U.S. Pat. No. 3,752,331 to Colburn. Here a complex scissors and screw mechanism is used to lift a platform which presumably could be placed under one edge of a load. In this instance, the problem of getting the mechanism out of the way of the lift truck is even more complex than in the previous example.